Domestic, commercial, and industrial wastewater is generally collected through an extensive network of sewers and transported to wastewater treatment plants, usually Publicly Owned Treatment Works (POTW). At a POTW, the wastewater is treated to a regulated level of cleanliness, and then subsequently discharged, or used as reclaimed water (e.g., for irrigation). The treatment processes can vary from location to location. Variation can depend on the age of the facility, the facility's capacity, local regulations, and the availability of capital. In the U.S., the infrastructure that leads to the production of biosolids includes an estimated 16,583 treatment works treating over 34 billion gallons of domestic sewage and other wastewater each day.
A typical POTW includes several stages, including preliminary treatment (screening and grit removal), primary treatment (sedimentation), secondary treatment (biological), and, in some cases, tertiary treatment. Solids removed in primary, secondary and tertiary treatment make up sludge or biosolid matter.
The U.S. Environmental Protection Agency (EPA) defines biosolids as solid, semisolid, or liquid residue generated during the treatment of domestic sewage in a treatment works. Due to increasingly stringent regulations to which municipal wastewater is required to be treated, the rate of production of biosolids has increased significantly.
Advancements in wastewater and sludge treatment technologies, increasingly stringent regulations, and population growth, have resulted in increased volumes of sludge which must be disposed of. More than $2 billion is spent annually treating and managing approximately 5.3 million dry metric tons of biosolids from POTW's in the United States. The rate of sludge production in the United States is forecast to increase to approximately 8.2 Mton/year (dry ton) by 2010, which is 19% increase from 1998. This trend indicates that the sludge production will continue to grow.
Traditional methods of sludge disposal include incineration, landfilling, and land application. Because of the reduced availability of land, the increased public concerns over food chain safety, EPA beneficial reuse policy, high disposal costs and harmful compounds, traditional disposal methods are no longer attractive.
Available techniques to reduce the volume of sludge include composting, dewatering, drying, and incineration. Dewatering is one of the most widely used techniques in the mass reduction of sludge from the wastewater treatment processes. However, final moisture contents of dewatered sludge cakes are generally higher than 75%.